This invention relates to multilayer ceramic capacitors and formulated ceramic powders for making capacitors that meet X7R performance characteristics of the Electronic Industries Alliance (xe2x80x9cE.I.A.xe2x80x9d), and in particular relates to a formulated ceramic powder from which there can be made a dielectric ceramic body that can be sintered at or below 1,025xc2x0 C. to permit the usage of a high silver content silver-palladium alloy electrode having an 85 weight percent or greater silver composition.
Efforts to produce low fired, high dielectric constant, X7R multilayer ceramic capacitors and formulated ceramic powder for making the capacitors are well known. For example, dielectric ceramic compositions that lead to a mature dielectric body with standard X7R performance characteristics and a high dielectric constant after firing or sintering at about 1,100xc2x0 C. are disclosed in several patents by one of the present inventors, Galeb Maher. Those patents include U.S. Pat. No. 5,010,443 that issued on Apr. 23, 1991 to Maher, U.S. Pat. No. 5,258,338 that issued on Nov. 2, 1993 to Maher, and U.S. Pat. No. 6,043,174 that issued on Mar. 28, 2000 to Maher et al., all of which patents are hereby incorporated herein by reference and all of which are assigned to the same assignee that is also the assignee of all rights in the present invention disclosed and claimed herein.
In the aforesaid U.S. Pat. No. 6,043,174 to Maher et al., a highly acceptable X7R formulated ceramic powder mixture is described that may be sintered after formulation at a temperature as low as 1,120xc2x0 C. in order to permit usage of electrodes consisting of 70% silver and 30% palladium. The ceramic powder mixture disclosed in the U.S. Pat. No. 6,043,174 Patent comprises at least ninety weight percent (xe2x80x9cwt %xe2x80x9d) pure barium titanate powder having an average particle size of from 0.4 to 0.7 microns, from 1.5 to 2.5 wt % of a cadmium silicate powder flux, a small amount of a grain growth inhibitor compound, such as 0.89 to 2.72 mole percent Nb2O5, and also about 0.2 to 1.0 mole percent calcium carbonate relative to the amount of barium titanate. Multilayer ceramic capacitors made from such a composition are currently being made and sold in the industry, and formulated ceramic powders made according to the disclosures in U.S. Pat. No. 6,043,174 are being sold by the assignee of the present invention, MRA Laboratories, Inc., of Adams, Mass., U.S.A.
Such ceramic powders and multilayer ceramic capacitors made from those powders have achieved high performance characteristics while reducing the amount of palladium within an electrode. As is well known, the high cost of palladium has made it desirable to decrease the amount of palladium and to increase the amount of silver in a silver-palladium alloy electrode. By decreasing the sintering temperature at which the ceramic powder is converted from a powder into a cohesive solid or mature body without melting the powder, the relative amount of silver in the silver-palladium electrode may be increased while the amount of palladium may be decreased. As indicated above, the ceramic powders disclosed in the U.S Pat. No. 6,043,174 Patent enabled sintering at a temperature of between 1,120xc2x0 C. to 1,150xc2x0 C. so that the amount of palladium in the electrodes could be reduced to 30 percent. It is understood in the art that the sintering temperature of the ceramic powder must be below the melting temperature of the silver-palladium electrode. Additionally, as the proportion of silver in the electrode is increased and the proportion of palladium in the electrode is decreased, the melting temperature of the electrode is decreased. Therefore, by lowering the sintering temperature of the ceramic powder, a greater proportion of silver and smaller proportion of palladium may be utilized in making the electrode to effect a significant cost saving due to the substantially higher cost of palladium compared to the cost of silver.
The aforesaid Patents also disclose further research efforts of the inventors herein to decrease costs and enhance performance characteristics of formulated ceramic powders for multilayer capacitors. For example, U.S. Pat. No. 6,043,174 also discloses that by an addition of a very small and critical quantity of a calcium compound such as calcium carbonate or calcium niobate to the start powder, a costly anneal step may be eliminated while maintaining a high insulation resistance in an accelerated life test with only a very small reduction of dielectric constant.
While known formulated ceramic powders and multilayer capacitors made from the powders demonstrate improved performance, nonetheless, existing powders still suffer from significant limitations. For example, it is known to use heavy metal oxides such as CdO in the powders as a flux. The advantages of use of cadmium silicate flux in multilayer capacitors were first disclosed by one of the inventors herein in U.S. Pat. No. 4,266,265 that issued on May 5, 1981 to Galeb Maher. However, such heavy metal compounds as cadmium oxide are increasingly considered hazardous materials, and hence they pose significant cost problems related to usage and disposal of components utilizing capacitors that include cadmium or other hazardous heavy metals. Additionally, as disclosed in the aforesaid patents, in sintering ceramic powders that include a cadmium silicate flux, a closed crucible must be used to contain any toxic cadmium fumes from contaminating persons in the vicinity of the crucible.
Accordingly, there is a need for a multilayer ceramic capacitor and powder for making the capacitor that does not include hazardous heavy metals, and that reduces costs of known capacitors.
The invention includes a dielectric ceramic powder mixture comprising at least ninety weight percent essentially pure barium titanate powder having an average particle size of from 0.2 to 1.2 microns; from 0.2 to 2.5 weight percent of barium lithium borosilicate (xe2x80x9cBLBSxe2x80x9d) flux; from 0.05 to 0.3 weight percent of MnCO3; from about 0.01 to 0.25 weight percent Co3O4; a grain growth inhibitor such as niobium oxide or a niobate compound yielding 0.4 to 1.50 weight percent Nb2O5; and, 0.4 to 1.4 weight percent of an additive selected from the group consisting of a rare earth oxide, a combination of rare earth oxides, yttrium oxide, or a combination of rare earth oxides and yttrium oxide, such that rare earth ions of the rare earth oxide have an average ionic radius of about 0.97 angstroms (xe2x80x9cAxc2x0xe2x80x9d), rare earth ions of the combination of rare earth oxides have an average ionic radius of about 0.97 angstroms (xe2x80x9cAxc2x0xe2x80x9d), yttrium ions of the yttrium oxide have an average ionic radius of about 0.97 angstroms (xe2x80x9cAxc2x0xe2x80x9d), or ions of a combination of rare earth oxides and yttrium oxide have an average ionic radius of xe2x80x9cabout 0.97 Axc2x0xe2x80x9d. The determination of an xe2x80x9caverage ionic radiusxe2x80x9d being xe2x80x9cabout 0.97 Axc2x0xe2x80x9d is based upon ionic radii measurements known in the art according to xe2x80x9cAhrensxe2x80x9d. (See, Ahrens, L. H. (1952), Geochim. Cosmochim. Acta 2., Pages 155-169. (Hereinafter, xe2x80x9cAhrensxe2x80x9d.)) (For purposes herein, the phrase xe2x80x9caboutxe2x80x9d a measurement, such as xe2x80x9cabout 0.97 Axc2x0xe2x80x9d includes plus or minus 20%, such as 0.97 Axc2x0 plus or minus 20%.
In the aforesaid U.S. patent application Ser. No. 10/036,205 some of the inventors of the present invention found that the rare earth oxide additive of gadolinium provided unexpectedly and highly desirable results in yielding a fine grain structure, and that when combined with Nb2O5 or a niobate compound as a grain growth inhibitor, a desirable core-shell structure known in the art was obtained that demonstrates excellent X7R performance characteristics after firing or sintering at temperatures as low as 875xc2x0 C. The rare earth oxide that demonstrated such desirable results is Gd2O3. The two gadolinium ions having a charge of plus 3 (xe2x80x9cGd+3xe2x80x9d) that combine with the three oxygen ions to form Gd2O3 (gadolinium oxide) each have a radius of about 0.97 Axc2x0. However, subsequent research has determined that additives including combinations of rare earth oxides also produce similar desirable results, provided the average ionic radius of the rare earth ions having a plus 3 charge is about 0.97 Axc2x0. Additionally, it has been found that yttrium oxide as an additive also promotes excellent performance characteristics, and the ionic radius of yttrium ions is within plus or minus 20% of 0.97 Axc2x0.
For example, a combination of 66% dysprosium oxide (Dy2O3) and 34% neodymium oxide (Nd2O3) produces an average ionic radius of the plus 3 charged rare earth ions of about 0.97 Axc2x0. The ionic radius of Dy+3 is 0.92, and the ionic radius of Nd+3 is 1.04, so that for a combination of 66% Dy+3 and 34% Nd+3, the average ionic radius of the plus 3 rare earth ions is about 0.97 Axc2x0.
Additional research provided further improvements in the dielectric ceramic powder by providing an enhanced range of weight percentages for BLBS flux of about 1 wt %; by showing that optimal performance of the BLBS flux may be obtained by adjusting the content of lithium silicate to about 50 molar percent of the BLBS; by demonstrating that addition of between 0.1 to 0.3 wt % of silver or between 0.1 to 0.3 wt % of copper also improved performance; by demonstrating substitution of a molar equivalent of tantalum oxide for the niobium oxide also provided adequate performance of the powder; and, by establishing that a partial substitution of the MnCO3 with between 0.1 to 0.2 wt % Co3O4 also provides for enhanced performance of the ceramic powder.
An additional aspect of the invention includes a method of making the above described preferred dielectric ceramic powder. Although good ceramic capacitors have been made by simply batching all the non-barium titanate minor additives with barium titanate during preparation of a capacitor paint slurry, further improvements in the properties and lowering of the sintering temperature were achieved when all the minor additives are mixed separately and pre-reacted at relatively low temperature. Preparation of the ceramic powder by this enhanced pre-reacting method includes the steps of: intensively wet milling the minor additives including Nb2O5, the described rare earth oxides, BLBS flux, Ag, MnCO3 and Co3O4 to achieve a particle size below 1.0 micron and preferably around 0.5 to 0.6 microns; drying the wet milled minor additives at about 150xc2x0 C.; then, granulating and mildly calcining the dried, wet milled mixture of minor additives at about 500 to 600xc2x0 C.; then mixing the calcined minor additives with the barium titanate. It has been discovered that this method of making the enhanced ceramic powder provides for a powder with improved performance characteristics because complex intermediate phases are formed by the described pre-reacting process.
Another aspect of this invention includes a method for making a multilayer ceramic capacitor body that satisfies X7R performance characteristics that includes the steps of forming a slurry of the above described dielectric ceramic start powder mixture by a dispersion of the mixture in a binder-solvent-dispersion system; preparing layers of the slurry; drying the layers; forming a stack of a plurality of the layers and interleaving patterned films of silver-palladium electrode ink between successive adjacent pairs of layers; and then heating to mature the stack of layers by sintering in open air at a temperature of between 875xc2x0 C. to 1,025xc2x0 C. for between 3 to 5 hours to produce a dense ceramic capacitor body with buried electrodes.
The multilayer ceramic capacitor made by that method demonstrates the aforesaid favorable X7R performance characteristics of dielectric constant (xe2x80x9cKxe2x80x9d) greater than 2,500, a smooth temperature coefficient of capacitance (xe2x80x9cTCCxe2x80x9d) wherein the K will be within +/xe2x88x9215% between xe2x88x9255xc2x0 C. to 125xc2x0 C., a dissipation factor (xe2x80x9cDFxe2x80x9d) of less than 2.5 percent, an insulation resistance greater than 100 ohm-farads at 125xc2x0 C., and long term stability during a life test at elevated temperatures and voltages.
It is therefore a purpose of this invention to provide a very low fired, high dielectric constant ceramic capacitor and the powder for making the capacitor that includes a gadolinium oxide or other rare earth oxide, or combination of rare earth oxides, or yttrium oxide, or a combination of yttrium oxide and rare earth oxides, as an additive that also includes no heavy metals. The resulting powder and ceramic capacitor has smooth X7R performance characteristics, may be fired or sintered at temperatures between 875xc2x0 C. to 1,025xc2x0 C., and includes no hazardous waste products, thereby minimizing manufacture and disposal costs, while permitting usage of a high silver content silver-palladium electrode having a sliver content in excess of 85 percent.
These and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawing.